Asthma is a growing problem, and outdoor allergens play a role in exacerbation of many cases. A clearer understanding of this role and its magnitude, and a means of controlling the effects of outdoor allergen exposures are needed. We propose Poisson time-series and conditional panel studies to test these hypotheses: 1) the incidence of acute asthma attacks, as measured by urgent care inhalation treatments and hospitalizations for asthma, has a dose-dependent relationship with exposure to specific outdoor allergens; 2) specific sensitization to outdoor allergens is a risk factor for having an acute asthma attack; and 3) exposure conditions that lead to acute asthma attacks can be forecast, creating an opportunity to reduce asthma morbidity and mortality by targeted pretreatment and/or exposure controls. We will test Hypothesis I using Poisson time-series analysis of daily outdoor allergen levels and urgent asthma treatments in a large HMO population over 4 years, controlled for air pollution, and meteorological conditions. We will test Hypothesis 2 in a subset of 1,000 of the acute asthma patients by skin testing them with standard allergen preparations and with extracts of air samples. Analyses will be controlled for age, total IgE level, indoor allergen sensitivity and exposure, time spent outdoors, and occupational and cold air exposures. Finally, we will develop models to forecast outdoor allergen exposure, and validate them using general cross-validation techniques. We have demonstrated that this study design is feasible in preliminary work: a) showing a dose-dependent relationship between both total pollen and urgent asthma treatments, b) recruiting and skin testing a small panel of Fallon patients with extracts of outdoor air samples c) generating a predictive model for ragweed pollen. This work demonstrates that we have access to useable clinical data, that we can recruit and test patients, that the study design has sufficient power to be definitive, and that there is a reasonable likelihood that the hypotheses are true. This study will lay a foundation for controlling acute asthma exacerbations caused by outdoor allergens, by identifying the specific allergens responsible, the patients at risk, and providing a means of forecasting hazardous exposure that can provide a focus for targeted prophylactic therapy.